In 1781 Charles Messier published a catalogue of 103 'smudges of light' - nebulae. At the time no one knew what nebulae were nor how far away they were. In due course telescope builder and astronomer William Herschel (1738-1822) recorded 2,500 nebulae. He was of the opinion nebulae were stars in the early phase of their life and like all other stars existed within the Milky Way. However Emanuel Kant believed nebulae were collections of stars beyond the Milky Way. Nebulae are often referred to by Messier numbers e.g. M31 being Andromeda one of our nearest extra gallactic neighbours.
To help decide what nebulae were, astronomers designed and built bigger more powerful telescopes, in particular:
- Lord Rosse, 1845 1.8m reflector, Ireland.
- Georg Hale (1868-1938), 40in refractor Yerkes, 60in Hale reflector Mount Wilson, 100in Hooker reflector Mount Wilson, 200in reflector Mount Palomar.
In 1920 the great debate on the nature of nebulae brought Harlow Shapley (Milky Way) up against Herber Curtis (independent galaxies). But the matter was resolved by techniques actually measuring the distances to nebulae. But how could this be achieved?
Henrietta Leavitt (1868-1921) studied Cepheid Variable (CV) stars. These ramped up brightness relatively quickly and then dimmed before repeating the cycle. Leavitt asked could there be a relationship between the period and the intrinsic brightness of Cepheids? She then had the bright idea of measuring CVs which would be at a similar distance from Earth - e.g. those lying in the Small Maganellic Cloud. Leavitt discovered a relationship between the true luminosity and the period of the apparent brightness. So if two CVs in different parts of the sky had the same period, but one was nine times fainter than the other, it must be three times further away.
However although astronomers could measure relative distances between two CVs, it required an absolute distance measurement to just one CV to allow all CVs distances to be determined. This was later accomplished by Shapley and Herzsprung.
In 1923, Edwin Hubble using the 100in Hooker telescope set about finding a CV in the Andromeda nebula. He found one with a period of 31.41 days i.e. 7000 times as bright as the sun. The CV and hence Andromeda seemed to be 900,000 light years away. Given the Milky Way was about 100,000 light years across Andromeda must be a separate galaxy.